1. Field of the Invention
The present invention relates to a traveling-wave tube, and more specifically to a delay line of the coupled-cavity type employed in a traveling-wave tube and having a high cooling characteristics.
2. Description of related art
At present, various traveling-wave tubes have been used in the field of telecommunication and broadcasting. In general, a traveling-wave tube comprises an evacuated envelope as of copper, containing an electron gun assembly at one end thereof for forming and projecting a beam of electrons over an elongated beam path to an electron collector electrode at the opposite end of the envelope. A slow-wave circuit or delay line is arranged along the beam path intermediate the electron gun and the beam collector for electromagnetic interaction with the beam. Input microwave signals to be amplified are applied to the upstream end of the delay line, and amplified output signals are extracted from the downstream end of the delay line. A periodic permanent magnetic arrangement is coaxially disposed to surround the envelope for producing axially arranged periodic magnetic fields within the envelope so as to focus the beam through the delay line.
The delay line employed in the traveling-wave tube as mentioned above is broadly divided into a helix slow-wave circuit and a coupled-cavity delay line. Referring to FIGS. 1 and 2, there is partially shown a typical example of the conventional coupled-cavity delay line for the traveling-wave tube. The shown delay line comprises a plurality of coupled cavities and a series of disks of magnetic material or pole pieces 10 aligned in parallel relation to each other along a common axis O--O which coincides with the axis of propagation of the electron beam. The delay line also includes a series of tubular members 11 made of for example copper, located coaxially to the common axis O--O with each of the tubular members being interposed between a pair of adjacent pole pieces. Each of the tubular members thus cooperates a pair of pole pieces located at opposite ends thereof to confine one cavity. Therefore, the pole pieces 10 form the wall which is common to two adjacent cavities 12. Each pole pieces 10 is pierced by two apertures 14 which provide a coupling between cavities. The pair of apertures are symmetrical with respect to the axis of the line O--O. In addition, each pole piece 10 is also pierced by an aperture 16 through which the electron beam is intended to pass. The aperture 16 is located at the center of the pole piece. In addition, the aperture 16 is usually of circular shape and surrounded by a coaxial annular flange designated as a drift tube 18. This drift tube is brazed to the pole piece 10. An electron-beam focusing device surrounds the cavities and is constituted by an alternate arrangement of permanent magnets 20 and pole pieces 10. Furthermore, for forced air cooling, the delay line includes a number of heat radiating or releasing fins 22 extending radially from a periphery of each pole piece 10 in equal angular intervals.
In operation, the drift tubes 18 are heated by a so-called body current and an induction current. The heat of the drift tubes 18 are transferred to the associated pole pieces 10 and then dissipated from the radiator fins 22.
In general, the larger the mean RF power becomes, the more the amount of heat generated in the drift tubes becomes. On the other hand, the pole pieces 10 are ordinarily made of soft iron to constitute magnetic circuit in cooperation with the periodic permanent magnets 20. The soft iron has fairly low heat conductivity, and therefore, if the heat of the drift tubes is increased, the pole piece cannot effectively transfer heat from the drift tube to the radiator fins 22. As a result, local overheating of a drift tube will often occur, which will have serious consequences such as generation of gas in the cavities and even cause destruction of the traveling-wave tube as a result of melting of the drift tube.
For overcoming this problem, U.S. Pat. No. 4,471,266 proposes installation of heat pipes actually extending through a series of pole pieces, so that the drift tubes are forcedly cooled via the associated pole pieces by a cooling liquid flowing through the heat pipes. However, this cooling is yet not sufficient, because the heat generated in the drift tube is transferred through the iron pole piece to the heat pipes. In addition, this cooling system is complicated in structure, and further has a latent danger such as cooling liquid leakage and damage of the heat pipes which would be caused when the cooling liquid is frozen.